In International Patent Application PCT/GB94/00405 (Street) apparatus is described in which a stereoscopic pair of two-dimensional images are combined with the aid of an optical system, so that each eye of an observer sees a different perspective of an object field, but in the same location. This causes the brain to fuse these perspectives into one three-dimensional image. For such fusion to be natural, it is important that the binocular disparity between corresponding points in each of these images is the same for points at a given distance, and that there should be little or no vertical disparity for such corresponding points. A typical stereo-endoscope comprises two optical telescopes, each having an objective, an image relay system, and an eye piece or camera lens. When video cameras are employed to present the images on a stereo monitor, the eye piece becomes the final lens in the chain and forms a suitably scaled image on the CCD sensor of the video camera. In most endoscopic designs, the objective lens has a very short focal length, providing a substantial field of view (say 60.degree.). The camera lens is typically of longer focal length, and the relay system which may comprise a coherent fibre bundle, a series of discrete lens elements, or a graded index lens system, provides a 1:1 magnification of the image formed by the objective, at the object plane of the camera lens. In many designs, the objective lens (typically comprising 2 or 3 elements) will image a flat square grid onto the CCD with some apparent distortion. Equally spaced points, further from the centre are seen as closer together than such points at the centre of the field. There are prior art implementations of stereoscopic viewing apparatus. Two examples are provided here. In International Patent Application PCT/US93/12306 (Kempf) various embodiments of a twin axis endoscope are described. All of these embodiments utilise one or more optical components to converge the two optical axes to cross at a common point at some predefined distance in front of the distal end of the scope. In International Patent Application PCT/US94/11089 (Taylor et al.) a single wide angle lens arrangement is placed in front of a binocular viewing system in order to allow stereoscopic viewing through a small opening at a reasonably wide angle.
When two optical systems are combined side by side in a stereo-endoscope, any point at a finite distance from the distal end of the scope and at the centre of the field of view of one telescope, will be observed at some distance from the corresponding axis of the other telescope. It will be apparent that the spacing of any two points will typically be different for each optical system. This difference can be quite severe towards the edges of the useful field of view. It is found that, deliberate lateral displacement of each CCD, a natural consequence of defining a point at a finite distance of a few centimeters from the objectives as the central point of convergence for the stereo system, accentuates this asymmetry. Neither of the prior art examples referred to above addresses the problem of cross distortion between the two images which are provided to the observer. This invention overcomes the problem by utilising the asymmetric properties of an additional optical element when acting in combination with the two juxtaposed optical systems.